Chemical reactions proceed from the initial reactants to the final products though various intermediates, and a proper understanding of such short-lived intermediates is highly desirable in many applications. In order to directly observe ultrafast transient processes such as chemical reaction intermediates and energy transfer in molecular, semiconductor and other systems, it is desirable to obtain broadband optical spectra in the low energy, e.g., 1.5 to 15 micron infrared, region of the spectrum. Picosecond or femtosecond pulsed lasers are available to initiate such an energetic process, followed by a measurement of the resulting transient changes in the optical properties of the system as a function of time.
Broadband visible probe pulse detection for observing transient energy transfer, molecular rearrangement and other chemical and physical processes has been accomplished in the past through the use of conventional spectrographs and optical multichannel analyzer (OMA) detectors. Intensified vidicons (ISIT) or linear array reticon detectors are extremely sensitive, approaching single visible photon detectability, and have sufficiently close-spaced pixels to obtain relatively high spectral resolution. Such detection schemes have been used to obtain molecular transient adsorption spectra directly in the optically visible range by picosecond or femtosecond continuum pulse generation. B. I. Greene, R. M Hochstrasser, and R. B. Weisman, J. Chem. Phys., 70, 1247 (1979); C. V. Schank, R. L. Fork, C. H. Brito Cruz, and W. Knox, in "Ultrafast Phenomena V", G. R. Fleming and A. E. Siegman, eds. (Springer-Verlag, N.Y. 1986), pp. 179-181.
Because of the lack of suitable multichannel IR detectors, measurements of transient adsorption spectra in the mid-infrared, i.e., from 1000 to 4000 cm.sup.-1, has involved reliance on scanning an independently tunable narrowband probe pulse, H. Graener, R. Dohlus, and A. Laubereau, in the Proceedings of Ultrafast Phenomena VI Conference, Kyoto Japan, July 1988, pp. 304; Chem. Phys. Lett., 140, 306 (1987), nonlinear frequency upconversion of an independently tunable diode laser, J. N. Moore, P. A. Hansen, and R. M. Hochstrasser, Chem. Phys. Lett., 138, 110 (1987), or the shifting of a nanosecond broadband dye laser into the infrared and then frequency shifting back into the visible for OMA detection. D. S. Bethune, A. J. Schell-Sorokin, J. R. Lankard, M. M. T. Loy, and P. P. Sorokin, in "Advances in Laser Spectroscopy", B. A. Garetz and J. R. Lombardi, eds. (Wiley, N.Y. 1983), Vol. 2. The latter method employs stimulated electronic Raman scattering in cesium or rubidium heat pipes and has recently been applied to picosecond and shorter time-domain experiments. J. H. Glownia, J. Misewich and P. P. Sorokin, Opt. Lett., 12, 19 (1987); Chem. Phys. Lett., 139, 491 (1987); M. Berg., A. L. Harris, J. K. Brown, and C. B. Harris, Opt. Lett., 9. 50 (1984). Prior approaches to this type of spectroscopy have involved scanning a narrow band IR probe laser with signal averaging to obtain a "point by point" spectrum of the ultrafast optical transient. In a similar fashion, a narrowband tunable continuous wave diode laser is upconverted at high repetition rate by an ultrashort pulsed laser to obtain the IR spectrum while scanning the diode laser throughout the infrared.
Previously applied technology of the type broadly related to the present invention involves shifting broadband visible dye lasers or ultrashort continuum in hazardous and highly inefficient metal vapor ovens into the infrared. Using this prior approach, the broadband infrared is then passed through a sample and thereafter upconverted back into the visible by a second metal vapor cell. The latter method has been successful only for long duration pulsed applications (nanosecond) and in a very limited spectral range (2.0 to 2.5 microns) when using femtosecond pulses. This method also requires elaborate technology to work in common laboratory environments.
There is, therefore, a need for a simple, compact spectrometer based on commercially available lasers, optics, spectrographs, and visible multichannel detectors to efficiently produce broadband IR and upconverted single-shot transient IR spectra.